Sea level influenced tropical climate during the last ice age

In a new study, Pedro DiNezio of the University of Hawaiʻi at M&#257noa International Pacific Research Center and Jessica Tierney of Woods Hole Oceanographic Institution investigated preserved geological clues (called “proxies”) of rainfall patterns during the last ice age when the planet was dramatically colder than today. They compared these patterns with computer model simulations in order to find a physical explanation for the patterns inferred from the proxies.

Their study, which appears in the May 19, 2013 online edition of Nature Geoscience, reveals unique patterns of increased and decreased rainfall over the Indo-Pacific warm pool, the vast pool of warm water stretching along the equator from Africa to the western Pacific Ocean. It also shows that they were caused by the effect of lowered sea level on the configuration of the Indonesian archipelago.

“For our research, we compared the climate of the ice age with our recent warmer climate,” explains lead-author DiNezio. “We analyzed about 100 proxy records of rainfall and salinity stretching from the tropical western Pacific to the western Indian Ocean and eastern Africa. Rainfall and salinity signals recorded in geological sediments can tell us much about past changes in atmospheric circulation over land and the ocean, respectively.”

Adds co-author Tierney, “Our comparisons show that, as many scientists expected, much of the Indo-Pacific warm pool was drier during this glacial period than today. But, counter to some theories, several regions, such as the western Pacific and the western Indian Ocean, especially eastern Africa, were wetter.”

In the second step, the scientists matched these rainfall and salinity signals of those two time periods with simulations from 12 state-of-the-art climate models that are used to also predict future climate change. The results revealed that the dry climate during the glacial period stems from greatly reduced convection over a region of the warm pool called the Sunda Shelf. Today the shelf is submerged beneath the Gulf of Thailand, but was above sea level during the glacial period, when sea level was about 120 meters lower.

“The exposure of the Sunda Shelf greatly weakens convection over the warm pool, with far-reaching impacts on the large-scale circulation and on rainfall patterns from Africa to the western Pacific and northern Australia,” explains DiNezio.

“Our research resolves a decades-old question of what the response of tropical climate was to glaciation,” concludes DiNezio. “The study, moreover, presents a fine benchmark for assessing the ability of climate models to simulate the response of tropical convection to altered land masses and global temperatures.”